The lactoperoxidase macromolecular probe system will be used to study the relationship between membrane structure and erythrocyte aging "in vivo." The erythrocyte membrane will be labeled with I125 using the lactoperoxidase catalyzed iodination. These cells will be injected back into an animal and the survival of the labeled components studied as a function of time. In other experiments an attempt will be made to separate cells as a function of age by differential centrifugation and other procedures. The separated cell populations will then be investigated to determine whether there are alterations in arrangement of membrane proteins as a function of age. In companion experiments "in vitro" aging will be studied in a similar manner. Abnormal cells such as those obtaind from individuals with spherocytosis, paroxysmal noctural hemoglobinemia, etc., will be investigated.